1. Field of the Invention
This invention relates to instantaneous floating point amplifiers which amplify a fluctuating input signal to a level within preselected limits.
2. Description of the Prior Art
In seismic exploration, sound waves are commonly used to probe the earth's crust as a means of determining the types and location of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by passing sound waves through that medium. In the reflection seismic method, sound waves or impulses are generated at a transmission point at or near the earth's surface, and sound waves reflected from subsurface reflecting boundaries are received at one or more receiving points. The received waves are detected by seismic detectors, e.g., geophones, which generate electrical signals at their outputs. Information relating to subsurface formations is contained in these signals, and they are recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas.
In a typical seismic field recording system the seismic detectors are arranged in arrays or nests. The outputs of these arrays are time-division mulitplexed, amplified, digitized, and recorded. In a typical time-division multiplexed system, the output of each array is sampled once per sample time, and it is common for the sample time to be 1 millisecond, 2 milliseconds, or 4 milliseconds.
The amplification of the output of a seismic detector array is complicated in that the amplitude of the input sound wave varies as a function of time. Additionally, the amplitude of the reflected sound waves decreases with time, because recording is typically continued after the generation of input sound waves is terminated. Accordingly, it is undesirable in seismic operations to utilize an amplifier having a fixed gain.
The amplifier which has commonly been utilized to amplify the output of a seismic array is known as an instantaneous floating point amplifier. The gain of this type of amplifier varies depending upon the magnitude of the input signal, and the amplifier is usually designed to apply a gain to this input such that the amplifier output, when sampled, is at a level within preselected limits. Typically, the amplifier is designed to amplify the input signal to a level between one-half and full scale output of the amplifier.
A typical instantaneous floating point amplifier includes a plurality of cascaded amplifier stages, and the number of stages and the gain of each stage determine the maximum gain that the amplifier can apply to the signal presented at its input. A given stage of the amplifier may or may not be used, depending upon the amount of gain that must be applied to the input signal to amplify it within the preselected limits.
A typical instantaneous floating point amplifier also includes control circuitry which determines, for each input signal, those stages of amplification which are required to amplify the input signal to within the preselected limits. This control circuitry typically generates a gain word which is representative of which stages of the amplifier are presently being utilized to amplify the input signal. Of course, as the number of stages of amplification in the amplifier increases, the generation of the gain word becomes more complex. It is, therefore, desirable to minimize the number of stages of amplification which are required to implement a given amplifier.
Another problem with prior art instantaneous floating point amplifiers is that each stage of amplification has an inherent offset voltage which is amplified together with the input signal. Since it is imperative that the ultimately recorded data be as uncorrupted by noise as possible, this amplified offset voltage must be removed from the amplifier output before later processing begins. This technique of removing the offset voltage has commonly been referred to as "nulling the amplifier."
Prior art amplifier systems typically employ rather complex circuitry to correct for the offset or employ a nulling technique which disables the amplifier for nulling while data is being received. This later techinque can, of course, result in a loss of valuable data.
The instantaneous floating point amplifier of the present invention overcomes these and other shortcomings of the prior art in the manner hereinafter described.